Various types of mobile radio communication system are known, and they differ essentially in geographical coverage, in the services they offer, and in the cost of these services.
Thus, there are public systems, in general cellular systems, having relatively large geographic coverage (typically a region, a country, or a continent), making a relatively large amount of mobility possible, but giving rise to relatively high call costs.
In such public cellular systems, and as shown in FIG. 1, mobile stations MS are in communication with a cellular public land mobile network PLMN connected to external networks, and in particular the public switched telephone network PSTN, and including in conventional manner a base station subsystem BSS and a network and switching subsystem NSS. For a fuller description of a cellular system, such as the global system for mobile communications GSM in particular, reference can be made for example to the book by M. MOULY and M. B. PAUTET, entitled "The GSM system for mobile communications", (Palaiseau, France, 1992, ISBN: 2-9507190-07-7).
Private systems are also known having relatively small geographic coverage, allowing relatively little mobility, but also giving rise to relatively low call costs. Such private systems can themselves be either cordless systems of the kind typically used residentially, or systems of the kind typically used in offices, and in particular made up of minicells that are much smaller in size than the cells of public systems.
In such private system, and as also shown in FIG. 1, mobile stations MS are in communication with private base stations PBS, themselves directly connected to an external network such as the PSTN, in particular.
There exists a need to be able to use a single mobile station to access one or the other of such systems, i.e. the public system or the private system, either at the choice of the user, or else more automatically with selection being performed by the systems themselves. "Multimode" mobile stations have thus been developed, which can be more or less complex depending on which functions are suitable for sharing between public and private modes, and also depending on how complex the private base stations are.
The base stations can be greatly simplified by providing for a common frequency band for both systems.
Nevertheless, under such circumstances, a problem arises in that if the mechanisms for allocating radio resources in these various systems are not coordinated, then interference is likely to occur between systems in an overlap zone, thereby degrading call quality.
It is recalled that in a public cellular system such as that shown in FIG. 1, it is the BSS entity of the PLMN that controls allocation of radio resources. More precisely, the BSS selects these resources from a set of resources allocated to a given cell (applying principles for sharing resources between cells in a cellular system), with the given cell being selected or reselected in co-operation with the mobile station MS.
In contrast, in a private system, the radio resources are determined in cordless systems in a non-centralized manner for each base station and mobile station pair, and optionally implementing frequency hopping techniques in order to limit interference within the system; in practice systems other than cordless systems, radio resources are determined in a more centralized manner at the private base station, but the problem mentioned above of interference between systems remains unsolved.
One solution to that problem of interference between systems would be to reserve for each system a distinct portion of said common frequency band, or for better spectrum usage, to reserve within each cell of the public cellular system a set of frequencies for a private system located within the cell, said frequencies being unattributed in the public system for the cell and for the adjacent cells, but being suitable for reuse in cells that are further away.
Such fixed distribution is nevertheless not optimum when system reconfiguration becomes necessary, and in addition it does not constitute an effective way of sharing resources.
A better solution consists in integrating resource management of the private system(s) in that of the public system, i.e. to manage and allocate radio resources dynamically for the various possible modes of operation, private or public, of the mobile stations.
Such a solution also has the advantage of making centralized allocation of radio resources possible in a private system constituted by a cordless system, thereby avoiding interference within such a system more effectively.
Nevertheless, such a solution gives rise in turn to the problem of finding the best way of informing mobile stations of the resources that have been dynamically allocated to them in this way for use in private mode.
It can thus be envisaged that such information could be transmitted to private base stations via the PSTN. Such a solution nevertheless suffers from the drawback of being relatively complex to implement.
It can also be envisaged that such information could be transmitted to the mobile stations over a common channel (or broadcast channel) of the public system, for example the broadcast control channel BCCH of the GSM system, where said channel is carried by a so-called "beacon" frequency and is used to broadcast general information specific to the system.
Nevertheless, such a solution suffers from the fact that most of such broadcast channels are already reserved for other uses. Furthermore, it is liable to give rise to delays in implementing algorithms for selecting or reselecting cells as implemented by the mobile stations, particularly since these broadcast channels have relatively low repetition rates within the frame and multiframe structures used for conveying them, and that would degrade quality of service.
Document EP 0 802 692 describes a system in which such information is transmitted to mobile stations by means of the short message service SMS. Such a solution has the particular drawback of requiring specific resources to be allocated for such transmission, and thus in particular of making non-optimum use of the radio resources available.